China is a major petroleum import country, and about 50 percent of petroleum is imported each year. Besides the majority of the petroleum resource is used as fuels for production and refining, there is also a considerable part of the petroleum resource used for the production of chemicals.
With the gradual depletion of petroleum resources, the focus of energy research has been shifted to biofuels in the whole world. In addition to the clean energy sources such as methane and hydrogen, biofuels-butanol wins the favor of more people and is known as the third-generation biofuels, because of its advantages, such as capable of being mixed with gasoline in any ratio, not requiring any reconstruction for vehicles, having high economic value, effectively improving the fuel efficiency and mileage of vehicles, and so on. At the aspect of bio-based chemicals, attention has been widely paid to some of the key platform compounds, such as 2,3-butanediol currently, which as an additive, can be widely applied to inks, cosmetics, lotions, plasticizers, drug and other industries, but also has the performance of liquid fuels. In addition, lactic acid, as an important chemical, is also widely used in the processing of food, pharmaceutical, cosmetics, chemical materials and agricultural products. At present, most of these bio-based products are produced from corn, wheat and other starch materials as the major raw materials through a saccharification and fermentation process. However, the production of biofuels from food supplies not only cannot meet the needs of the community, but also will endanger food safety. It was reported by some researchers that even if all the corn and soybeans grown in the United States were used for the production of bio-energy, only 12% of U.S. gasoline demand and 6% of U.S. diesel oil demand could be satisfied. In fact, corn and soybeans cannot be all used to produce biofuels, because their priority use is for foodstuff, feedstuff and other economic needs.
China has very rich straw resources. Its annual output is about 700 million tons. However, most of the straws have not been effectively utilized till now. If we can use bio-refining technologies to produce chemicals, materials and fuels from straws, as the main raw material, the petroleum import pressure can be sufficiently alleviated in China. Therefore, exploring the fermentation of non-foodstuff plants such as straws, as the raw materials is an important route to achieve the sustainable production of bio-based products.
The main components of the cell walls of straws include cellulose, hemicellulose and lignin. Lignin together with hemicellulose are filled as an intercellular substance between the microfine fibers in the cell walls, reinforce the cell walls of the wood tissue, and are also present in the intercellular layer to allow the adjacent cells to be bonded together. Cellulose, as a chained polymer of glucoside linked by beta-1,4-glycosidic bond, can be used for the production of all kinds of chemical and material products, and can also be biodegraded into glucose for the production of a variety of fermentation products. Hemicellulose is a general designation for a large class of polysaccharides having different structures, such as pentosan, poly-arabinose, polymannose, polygalactose, wherein the main component is pentosan. The degradation products of hemicellulose mainly include xylose, arabinose, mannose, and a small amount of glucose, can be converted by microbes into biofuels such as ethanol, butanol, methane and hydrogen, and can also be fermented into butanediol, xylitol, lactic acid and single-cell protein. Lignin, as an important chemical product itself, is a class of complex amorphous materials of phenylpropanes units linked by ether bonds and carbon-carbon-bonds.
Currently, the main problems up against the use of straw to produce biofuels include the high costs for raw material pretreatment and enzymatic hydrolysis, as well as the low utilization value of the raw materials. The reasons for that are shown as follows. First, people tend to only pay attention to the use of the cellulose that is difficult to be degraded in straw, but turn a blind eye to the hemicellulose resource in an amount of 25%-35% of straw. In fact, steam explosion pretreatment and dilute acid pretreatment can be used to effectively degrade and remove hemicelluloses but remain lignin and cellulose. Then after an alkali treatment, the lignin and cellulose can be obtained. Straw cellulose, due to its special chemical structure, has a great market value in the material product industry. However, in practice, people tend to degrade straw cellulose completely, while ignoring its functional features, thus resulting in the high degradation cost and low production value. In addition, the lignin in straw is often used as fuel for direct combustion. Nevertheless, in fact, the processed lignin can be made into a chemical raw material with a high added value.
Comparison of the disclosed invention patents is shown as follows. U.S. Pat. No. 2,481,263 discloses a process of direct fermentation of acetone, butanol and ethanol from pentose acid hydrolyzate. The key innovation of this invention is that fine iron powder and limestone are used for the detoxification of the acid hydrolyzate, then the xylose solution is fermented after flash-steam sterilization. In this patent, a great amount of iron powder and limestone are required in the detoxification process, and no comprehensive utilization of the raw materials is taken into consideration. Therefore, as the environmental pressure increases gradually, it cannot be used for industrial production apparently. U.S. Pat. No. 4,424,275 discloses a method of the continuous production using butanol, characterized in that butanol is first continuously extracted by a solvent extraction method, and then the extraction solvent is recycled in combination with distillation. Although this patent involves some innovation to the fermentation method, the issues regarding the raw materials are barely considered. U.S. Pat. No. 4,539,293 discloses a method of co-fermenting Clostridium pasteurianum and C. Butylicum, so as to improve butanol production and the proportion of the butanol in the solvent. However, no innovation was made to the raw materials for fermentation. U.S. Pat. No. 4,649,112 discloses a method of directly fermenting corn bran fiber or a mixture of corn bran fiber and xylan to obtain butanol by C. acetobutylicum. Although the pre-hydrolysis step is omitted in this method, the fermentation microbes mainly utilize the starch contained in the raw materials for fermentation, which also limits the types of raw materials for butanol fermentation. U.S. Pat. No. 4,777,135 discloses a method to promote butanol fermentation by adding a fluorocarbon compound in the butanol fermentation broth. However, this patent barely involves the source of the raw materials. U.S. Pat. No. 5,063,156 ameliorates butanol fermentation from the point of view of the fermentation methods, and improves the proportion of butanol in all the products during the butanol fermentation process by means of the combination of continuous fermentation and batch fermentation. Chinese Patent with Publication No. CN 101434968A discloses a method for producing fuel butanol from tapioca. Even though cassava is a non-food raw materials, the cultivation of cassava still requires vast lands. It was also reported that since the production of cassava alcohol began in Guangxi, the price of cassava has been highly raised, which causes the cost of cassava alcohol is higher than its market price. In this case, the same problem will also be faced by the production of butanol from cassava. Therefore, the exploration of butanol production from straw-cellulose raw materials will be a better production route. Chinese Patent with Publication No. CN101358218A discloses a method to produce pentose together with acetone, butanol and ethanol from straw. Chinese Patent with Publication No. CN101358214A discloses a method to produce furfural together with acetone and butanol from straw. In both of these patents, straw is used as the raw material; after hydrolysis, the hemicelluloses in the straw is converted into five-carbon sugar or furfural; the remaining hydrolyzate is, after treated, converted into glucose through enzymolysis; then fermentation is conducted to produce butanol. Although by this method butanol can be produced, now it seems that the degradation of cellulose to glucose requires cellulase, which involves a very high production cost. As a result, using this route to the production of butanol, the production cost is high. When the butanol is used as a fuel, the price cannot be accepted by the market.
Therefore, it is desired to find a cheaper and easier process for pretreating raw materials, so as to improve the efficiency of utilization and degradation of straw, improve product yield and reduce production costs at the same time.